How to Calculate Voltage Drop for LV Cable Runs

Why Voltage Drop Matters

Voltage drop is the reduction in voltage that occurs along the length of a cable carrying current. In a low voltage distribution system, excessive voltage drop means the equipment at the end of the run receives less than its rated supply voltage — leading to reduced motor torque, nuisance tripping of under-voltage relays, dimming of lighting circuits, and potential overheating as motors draw higher current to compensate.

Voltage drop is a sizing criterion that operates independently of ampacity. A cable may be fully adequate to carry the load current thermally (per ampacity derating) while still producing unacceptable voltage drop — particularly on long runs, motor feeders (see motor list for FLC data), and circuits in large industrial plants where equipment can be hundreds of metres from the distribution board.

The Basic Formula

For a resistive load on a three-phase circuit, the line-to-line voltage drop is:

• VD = √3 × I × R × L

For a single-phase circuit:

• VD = 2 × I × R × L

Where:

• VD = voltage drop (volts)
• I = load current (amperes)
• R = conductor resistance per unit length (Ω per metre or Ω per 1000ft)
• L = one-way cable length (metres or feet)

The factor of √3 for three-phase accounts for the phase relationship between conductors. The factor of 2 for single-phase accounts for the return path through the neutral.

For circuits where power factor and reactance are significant (larger conductors, longer runs, or heavily inductive loads), the full phasor formula applies:

• VD = √3 × I × L × (R cosθ + X sinθ) — three-phase
• VD = 2 × I × L × (R cosθ + X sinθ) — single-phase

Where X is the conductor reactance per unit length and θ is the load power factor angle. For most LV feeder calculations below 150mm², and where power factor is above 0.85, the reactance term is small enough that the simplified resistive formula gives adequate accuracy.

Conductor Resistance Values

Conductor resistance varies with material, cross-section, and temperature. The two most common references are:

• NEC Chapter 9, Table 9 — AC resistance in Ω per 1000 feet at 75°C conductor temperature for copper and aluminium conductors in conduit. Used for NEC-based design in North America.
• IEC 60228 — conductor DC resistance at 20°C (corrected to operating temperature per IEC 60287-1-1), corrected to operating temperature using a temperature coefficient. Used as the basis for IEC ampacity calculations per IEC 60364-5-52.

A selection of representative values at operating temperature:

• 4mm² copper — approximately 4.61 Ω/km (IEC) / 1.26 Ω/1000ft (NEC equivalent ~10 AWG)
• 16mm² copper — approximately 1.15 Ω/km (IEC) / 0.491 Ω/1000ft (NEC equivalent ~6 AWG)
• 50mm² copper — approximately 0.391 Ω/km (IEC) / 0.154 Ω/1000ft (NEC equivalent ~1 AWG)
• 120mm² copper — approximately 0.161 Ω/km (IEC) / 0.0608 Ω/1000ft (NEC equivalent ~4/0 AWG)
• 240mm² copper — approximately 0.0754 Ω/km (IEC) / 0.0258 Ω/1000ft (NEC equivalent ~500 kcmil)

Aluminium conductors have approximately 1.6× the resistance of equivalent copper cross-sections, requiring a larger conductor to achieve the same voltage drop.

Permitted Voltage Drop Limits

Neither NEC nor IEC mandates a single universal voltage drop limit — both give guidance rather than hard requirements, with the expectation that the engineer and project specification define the applicable limit.

• NEC (NFPA 70) Art. 210.19(A) FPN No. 4 and Art. 215.2(A) FPN No. 2 recommend that voltage drop on branch circuits not exceed 3%, and that the combined voltage drop on branch circuit plus feeder not exceed 5%. These are recommended fine print notes (FPNs), not mandatory code requirements.
• IEC 60364-5-52 Section 525 provides general guidance of 3–5% depending on the supply system and load type. Project specifications typically prescribe tighter limits — commonly 3% for lighting, 5% for power circuits, and 2.5% for sensitive equipment or instrumentation.
• Motor starting — a separate check is required for motor feeder circuits. The voltage at the motor terminals during starting must remain above a minimum level to provide adequate starting torque. Typically 80–85% of rated voltage during DOL starting is the accepted minimum. This often governs feeder sizing for large motors on long runs.